Extend helm-tags.el to support gtags

### Suggestion

Currently helm-tags.el only support etags. But in practice, gtags is more popular as it support not only finding definition as etags already do, but also cross reference (e.g. find the usage of a function/variable, etc). And gtags is the only tags-based solution I know that support cross reference. Also gtags use database for incremental update, which makes it the defacto solution for browsing code in gigantic project like linux kernel.

### Description

Extend helm-tags.el to have defcustom for which tags program to use. Or have another stand-alone `helm-gtags-select` just like the `helm-etags-select` we already have to use helm interface for selecting tags/references.

### Solutions

There is already [helm-gtags](https://github.com/emacsorphanage/helm-gtags) on melpa, unfortunately the package is out of maintenance and is now being deprecated.

Start by reading helm-tags.el and comparing the existing helm-etags-select flow with the deprecated helm-gtags package linked in the issue. Determine whether configurable tag programs or a separate helm-gtags-select entry point fits the current design. Done means Helm can select tags and references from gtags, including cross references, with the chosen interface integrated into helm-tags.el.

Written by the indexing model from the issue text.